This invention relates to pole changeable three phase winding in general and more particularly to a pole changeable three phase winding for a fractional pole pair ratio of the first and second numbers of pole pairs p.sub.1 and p.sub.2 according to p.sub.1 :p.sub.2 = (3m .+-. 1): 3n, with p.sub.2 = 3n is an integral multiple of the number of phases, where m and n are positive integers.
The step-wise change of the speed desired in three phase machines, particularly asynchronous squirrel cage motors, by suitably changing the effective number of stator poles, can be achieved at economically justifiable cost in two ways. The first way is to use a separate stator winding for each desired number of poles; this allows far reaching freedom in the design of the winding for practically any pole number ratio desired and makes possible, in particular, large steps in speed. Since only part of the slot cross section is available for each winding, relatively high copper losses, reduced torque and poorer cooling of the windings result. The utilization of such a machine is therefore greatly reduced. Also, the manufacturing costs for such a stator with separate windings are relatively high.
A better utilization of such electric machines can be achieved with a single, pole changeable winding for the different speed steps. For many pole number ratios, this is possible by using the principle of current reversal always in one half of the phase windings. For two speed steps with only six terminal points a separately changeable Y-point bridge is necessary, in addition to a three-pole double-throw switch. A special case of such pole switchable three phase windings is the most frequently used Dahlander circuit for a pole number ratio of 2:1.
Stator windings with a pole number ratio different from 2:1 have become known as so-called pole amplitude modulated stator windings or PAM windings (cf. H. Schetelig and R. Weppler, "Polumschaltbare Drehstrom-Kaefiglaeufermotoren mit PAM Wicklung" -Pole-switchable Three-Phase Squirrel Cage Motors with PAM Windings-ETZ-A Vol. 92 (1971), no. 10, pages 576 to 579, as well as the patent literature cited there). With the PAM windings also, each phase winding is subdivided in the middle and the poles are changed by always reversing the direction of the current in one half of the phase. In general, the number of individual coils for each pole and phase winding are different from pole to pole. In PAM windings, the coil arrangement of the three phase windings is also different if one of the numbers of pole pairs is a multiple of three. As a rule, two layer windings with coils of equal coil pitch are used for this purpose, the coil pitch usually being designed with the pitch diameter of the higher number of poles.
In these PAM windings, symmetry of the field shape is dispensed with and partly strongly developed undesired even harmonics and subharmonics must be tolerated. According to Sequenz, "Die Wicklungen elektrischer Maschinen" - The Windings of Electric Machines, vol. 3, Springer-Verlag, Vienna, 1954, perfect winding symmetry which is otherwise necessary in order to avoid such field harmonics, with the same number of coils per phase for each pole, has heretofore always been sought, which led to considerably more complicated windings with a relatively large number of terminal points and accordingly expensive switching devices.
Subharmonics and even harmonics of the field must be avoided as far as possible, especially in asynchronous machines with a relatively small air gap because of possible parasitic effects such as noise, vibrations, torque, harmonics, shaft voltages, etc. Except for a pole pair number ratio p.sub.1 :p.sub.2 = 2:1 (Dahlander circuit), this cannot be achieved, however, by using the principle of current direction reversal in one half of the phase winding (PAM windings). Rather, the individual winding branches must be regrouped circuit-wise for this purpose, changing the assignment to the three phase windings in cyclically interchanged sequence, for which purpose considerably more than six terminal points and accordingly expensive switching devices are required, which, among other things, also follows from the cited book by Sequenz.
Thus, according to the German Pat. No. 656,277, at least twelve terminal points are required for windings that are pole changeable in the ratio 3:2, and still nine terminal points as well as corresponding switch-in means for a winding according to the German Auslegeschrift No. 1 022 306. In both cases, the individual winding branches of the symmetrically arranged three phase windings are regrouped, after the pole changing, with a cyclically interchanged phase assignment.
From the German Auslegeschrift No. 2 107 232, pole switchable three phase windings with only six terminal points are known, in which the pole changing is likewise accomplished by cyclical interchange of the phase assignment. However, here the individual winding branches, which are triple-Y-connected for both pole number stages, must be formed by coils or groups of coils with different numbers of turns. The necessary number of turns must be determined as a function of the desired number of poles from the spatial location of the coils via trigonometric functions, which is laborious and makes the fabrication of such windings considerably more difficult. Since only integral numbers of turns for the coils can be realized in practice, calculated values must be rounded off up or down in each individual case, and asymmetries remain in the parallel winding branches, so that circulating currents caused thereby can flow.
Internal equalization currents also occur, incidentally, in the winding according to the German Auslegeschrift No. 1 022 306, since the winding branches which are connected parallel in the pole number step with the triple delta connection have different phases.
A circuit similar to that from the German Auslegeschrift No. 2 107 232 is known from the journal "Revue Generale d'Electricite" 82 (1973), 5, pages 323 to 329. The 6/4-pole three phase winding discussed there with only six terminal points represents a special case of so-called phase-modulated windings, which normally require considerably more than six connecting ends. This three phase winding has a triple Y connected first winding part with separate Y points, which is effective, i.e., current carrying, for both pole numbers, and a second winding part which is connected to the ends of the first winding part which are tied together and which carries current only in 6 pole operation. As a result of the fact that phase windings of a 4 pole winding basically can not be subdivided into three parallel branches of equal design, a rotating field with a high harmonic content and pronounced subharmonics (v = 1/2, 5/2, 7/2 etc.) and even harmonic is generated in 4 pole operation in this known pole changeable three phase winding. Such harmonic fields affect the operating behavior adversely, and particularly in asynchronous machines with a small air gap, noise, vibrations, torque harmonics or shaft voltages can appear as parasitic effects.
With the pole changing based on an interchanged phase assignment of individual winding branches, an adaptation of the numbers of turns effective for the two numbers of poles is posible only by chording the coils to a relatively limited extent, which, however, is always accompanied by a degradation of the shape of the field curve. The coil pitch most favorable with respect to the field shape corresponds, in most cases, to the pole pitch for the higher number of poles. On the other hand, adaptation is facilitated in the case of PAM windings, since besides the possibility of coil chording, the variants with Y, delta, double Y and double delta connections known from the Dahlander circuit are available.